There are various acoustic techniques for estimating the number density (i.e. population size) of biological or particulate matter in the ocean. However, such techniques are typically only capable of estimating the order of magnitude of such populations, due to the highly random nature in which they are distributed.
The prior art techniques typically employ a single frequency pulsed echo sounder having transmit and receive capabilities. Biological or particulate matter in the ocean region through which the acoustic signal is transmitted scatter the signal. The receiver receives a backscatter signal containing both amplitude and phase information. However, the prior art techniques aforesaid utilize only the backscatter signal amplitude information, because the phase information contained in successive backscatter signals emanating from the randomly distributed target population is also randomly distributed. Accordingly, the prior art techniques aforesaid are unable to make effective use of the backscatter signal phase information.
It would be highly useful to employ the phase information contained in the received backscatter signals. Sonar signalling techniques for underwater speed measurement and echo location and other related techniques require precise determination of the delay between the time at which an acoustical signal pulse train is propagated into the water and the time at which the signal (or a backscatter reflection of the signal) is received. It happens that the phase angle of the received acoustical pulse, together with pulse amplitude information, permits more accurate time (i.e. signal propagation delay; and therefore distance) measurements to be made than those attainable by working with only the amplitude information contained in the received pulse. Thus, the problem is to measure accurately the phase angle of the received pulse relative to that of the transmitted pulse. The problem is compounded by the random target distributions of the sort presently of interest. As previously indicated, successive measurements of the phase of the backscatter signal produced by randomly distributed target populations are not individually useful because they are also randomly distributed.
The present invention provides a technique for coherently summing the full acoustical backscatter signals which are received. By separating preserving and summing the in-phase and quadrature components characterizing the received backscatter signals, one may simulate the backscatter signal which would have been obtained, had the target population been more densely distributed. More particularly, by utilizing an iterative process in which the in-phase and quadrature components characterizing successive backscatter signals are separately summed, one may simulate phase coherent signal reflections from the randomly distributed target population, notwithstanding the fact that individual received backscatter signals are incoherent with respect to one another. The technique can be shown to facilitate highly accurate determination of the mean spacing between the randomly distributed targets; determination of the density of the targets (and therefore determination of the number of targets); and, determination of the mean target strength.
It is expected that the invention will have particular application in relation to the location and identification of certain fish populations for commercial fishing and/or fisheries management conservation purposes. However, it is important to recognize that the technique is not restricted to situations in which the randomly distributed target population is in water, or even to situations in which the population is in a liquid. For example, it is expected that the technique will offer important advantages in the study of piped fluids such as oil or other commercially valuable substances. Accurate determination of the characteristics of randomly distributed populations of targets such as solid particles or gas bubbles contained in such fluids; namely, the mean target spacing, the target population density (and therefore the number of targets) and the size of the targets in a rapid, efficient manner would be highly useful in many applications where such measurements are not presently possible so far as the inventors are aware. The technique is also expected to offer advantages in certain electromagnetic applications, such as in rain-radar techniques.
SUMMARY OF THE INVENTION
In accordance with the preferred embodiment, the invention provides a method of simulating phase coherent signal reflections in a medium containing a random distribution of targets which are capable of scattering signals transmitted through the medium, provided that the energy of signals directly scattered by the targets is significantly greater than the energy of signals which are multiply scattered by the targets. A reference signal having a wavelength (".lambda.") which is small in comparison to the expected mean spacing between the targets is modulated and then transmitted through the medium. The targets produce scattered echoes of the signal. The scattered echo signals are received and in-phase and quadrature components of the received signals are then derived and separately summed and normalized, giving estimates of the mean In-phase (I), and Quadrature (Q) components. Groups of these cumulative estimates of I and Q are then used to fine their variance. A transformation of the number of independent realizations (n.sub.i), leads to a linear relationship between the variance and l/n.sub.i ; the slope of a line fitted to the resulting distribution can then be used to determine the number of targets in the scattering volume.
Having determined I and I, the corresponding amplitude A and phase .theta. can be found from the usual transformation to polar co-ordinates; i.e. EQU A=I.sup.2 +Q.sup.2 EQU .theta.=Arctan (Q/I).
Advantageously, target populations lying within selected regions of the target-containing medium are examined by delaying commencement of the scattered echo signal receiving step for a selected time interval after transmission of the modulated reference signal through the medium and then performing the scattered echo signal receiving step for a further selected time interval.
If the target-containing medium is water, and if the targets are fish, then the reference signal wavelength is preferably significantly greater than the expected mean length of the fish.
The invention also facilitates determination of the mean number of targets No (r) and the target number density p(r) within the scattering volume.